A young boy featured in a chilling new Islamic State execution video is feared to be the son of a runaway jihadi bride from London.

Scotland Yard was today working with Lewisham council officials amid claims that the child — dubbed Jihadi Junior — may be one of the sons of Grace “Khadijah” Dare, from south-east London.

Previously a devout Christian, Dare was among the first Britons to join IS and told of her wish to become the first UK woman to kill a Westerner.

The boy, dressed in military fatigues and a black bandana bearing the mark associated with IS, declares: “We will kill kuffar [non-believers]” in new footage in which five shackled men in orange jumpsuits are shot dead, after being accused of spying for Britain.

Dare, thought to be aged 24, fled to Syria in 2012 and married a Swedish Islamist fighter called Abu Bakr.

Last year she tweeted a photo of her then four-year-old son Isa smiling as he aims an AK-47 rifle. He bears a remarkable similarity to the child in the latest propaganda video.

Grace was a very quiet person but could be feisty... One day she was wearing Western clothes and then the next day she was in a hijab

A neighbour of Grace Dare

Isa has a younger brother — who would now be two or three — who his mother has referred to as a “mini-mujahid”, or holy warrior.

The boy in the 10-minute video, believed to be aged five or six, appears at the end of the footage.

A masked man, believed to be British, attacks David Cameron, labelling him “an imbecile” for launching air strikes in Syria, before killing a prisoner.

Four other men then kill one victim each.

He appears to be seeking to copy Londoner Mohammed Emwazi, “Jihadi John”, who took part in beheading hostages including Britons David Haines and Alan Henning, before being killed in a US drone strike in November.

Security services and police are examining the new video. Scotland Yard said: “The Met’s Counter Terrorism Command is assessing the content of videos posted online that appear to show the murder of hostages.

"The public is reminded that the dissemination of extremist material may constitute an offence under terrorism legislation.”

The presence of a masked jihadi with an apparently southern British accent in the latest IS murder video is undoubtedly disturbing. So too is the appearance of an English-speaking boy making death threats.

But ultimately, despite its vile content, the film released by IS yesterday communicates nothing new and seems more a sign of weakness than strength.

All of the key elements of the footage — the killing of alleged “spies”, the rant against the West and one of its leaders, in this case David Cameron, and even the presence of a child — have featured in IS videos before.

Nor is the role of a Briton at the centre of such barbarous activities any surprise. UK jihadis have long been at the heart of Islamic State’s murderous campaign in Syria and Iraq, with several already placed on the same drone “hit list” that led to the fatal strike against Londoner Mohammed Emwazi.

If not already on it, the masked jihadi in the new video, once identified, seems certain to be added to that list because of the explicit threats that he makes to this country.

Whitehall sources have already dismissed the video as an attempt to divert focus from IS setbacks in the Iraqi city of Ramadi and elsewhere and the latest in a long line of “shock” propaganda films featuring murders, as well as the destruction of historic sites, used by the extremists with now diminishing returns.

That analysis appears valid. It is the threats contained in the jihadi’s diatribe against Britain that are the video’s most telling content, not for their menace, but instead for their desperate attention seeking.

Using language described by a BBC presenter today as fit for “student politics”, the apparently British killer warns that this country will be invaded and placed under Sharia law by IS.

Mr Cameron is described as an “imbecile” while those who oppose IS are “fools”.

The jihadi appears to have forgotten one of the first rules taught to every school child: that resorting to insults is a sign of losing the argument.